Semiconductor device

ABSTRACT

A dynamic memory requires refreshing to retain data in its memory cells. This may cause access to the dynamic memory for purposes other than refreshing (external access) and access to it for refreshing to compete with each other, resulting in a performance deterioration. According to this invention, a pipelined dynamic memory (PDRAM) is used, and the pipeline frequency (CLK) of the pipelined dynamic memory is made higher than the frequency (CLK 1 ) of external access, and access required for refreshing is made to an unoccupied slot (a timing when any external access request is never issued) in the pipeline of the pipelined dynamic memory. This makes refreshing of the internal dynamic memory an internal operation, which eliminates the need to take refreshing into consideration at the time external access is made, leading to improvement in operating ease and speed.

FIELD OF THE INVENTION

The present invention relates to a dynamic memory and a semiconductordevice which uses it, particularly to a dynamic memory suitable for highspeed applications with low power consumption and a semiconductor devicewhich uses it.

BACKGROUND OF THE INVENTION

The operating waveforms of a conventional dynamic memory (hereinaftercalled DRAM) which stores data in its memory cells each consisting ofone n-MOS transistor and one capacitor are as shown in FIG. 2, forexample, according to the book about “VLSI memories” authored by KiyooItoh (published by Baifukan 1994, p.86). Here, in reading, after wordline WL is asserted to read the signal from a memory cell to the bitline BL, /BL, the sense amplifier is activated at a prescribed timingφAto amplify the signal on the bit line. As a result, when a row addressaccess time (tRAC) has elapsed after the start of the access, finaloutput of data occurs. A time for rewriting into the memory cell, tRAS,is required before a precharge time (tRP) is needed to precharge the bitline and the like.

The writing sequence is basically similar to the reading sequence; afterthe sense amplifier is activated, the bit line is activated according towrite data to write in a selected memory cell.

This type of dynamic memory needs refresh operation to retain the datain memory cells.

Conventional dynamic memories as mentioned above have the following fourproblems:

Firstly, for reading, the amplitude of the bit line must be large forrewriting into the memory cell. This means that the cycle time (tRC) asexpressed by tRAS+tRP must be long.

Secondly, for writing, non-selected memory cells should operate in thesame way as for reading, which also leads to a longer cycle time tRC asin the case of reading operation.

Thirdly, for the above two reasons, if the dynamic memory is fullypipelined, the pipeline pitch must be long.

Fourthly, due to the necessity for refreshing operation, access to thedynamic memory (external access) for purposes other than refreshing andaccess to it for refreshing compete with each other, resulting in aperformance deterioration.

SUMMARY OF THE INVENTION

To solve the above-mentioned problems, the present invention provides asemiconductor device which has: a memory circuit which includes pluralmemory cells provided at intersections of plural bit lines and pluralword lines; and an access control circuit which receives an externalcommand and an external address to read data from or write data to thememory circuit at the transition point of a first clock, and suppliesthem to the memory circuit as an internal command and an internaladdress to read data from or write data to the memory circuit, at thetransition point of a second clock whose frequency is higher than thatof the first clock. The access control circuit further has a refreshcontrol circuit which refreshes the plural memory cells at thetransition point of the second clock, a timing which does not allow theexternal command and the external address to be supplied.

Thanks to this configuration, in the memory circuit, even when memorycells require refreshing, the refreshing operation can be isolated fromexternal control as an internal operation so that it can be concealedfrom outside.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be described indetail based on the followings, wherein:

FIG. 1 illustrates an embodiment according to this invention;

FIGS. 2A and 2B are graphs showing operating waveforms of a conventionaldynamic memory;

FIGS. 3A and 3B show an example of operating waveforms of a dynamicmemory according to this invention;

FIG. 4 illustrates an embodiment of a dynamic memory based on a directsensing scheme according to this invention;

FIGS. 5A, 5B and 5C illustrate an embodiment of a pipelined dynamicmemory according to this invention and its operating waveforms;

FIGS. 6A, 6B and 6C show an embodiment of a pipelined dynamic memoryaccording to this invention, in which the write latency is identical tothe read latency, as well as its operating waveforms;

FIG. 7 shows an embodiment which has a forward circuit in addition tothe embodiment shown in FIG. 5;

FIGS. 8A and 8B show examples of usage of a dynamic memory according tothis invention in case where no cache memory can be used;

FIG. 9 illustrates an embodiment of a pipelined dynamic memory;

FIG. 10 illustrates an embodiment of a refresh-free dynamic memory whichhas an access control circuit to conceal refreshing operation fromoutside in addition to a pipelined dynamic memory PDRAM;

FIG. 11 is a timing chart for operation of the embodiment shown in FIG.10;

FIG. 12 illustrates an embodiment of a refresh-free dynamic memory incase where the frequency ratio of CLK1 and CLK2 shown in FIG. 10 is 3/2;

FIG. 13 is a timing chart for operation of the embodiment shown in FIG.12;

FIG. 14 illustrates an embodiment which has a clock generating circuitin addition to the embodiment shown in FIG. 10;

FIG. 15 illustrates an embodiment of a pipelined dynamic memory whichuses 3T memory cells; and

FIG. 16 illustrates an embodiment of a merged DRAM/logic LSI which usesa refresh-free dynamic memory according to this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of this invention are detailed next, referring tothe attached drawings. Non-limitatively, in these embodiments, circuitelements which constitute blocks are formed on a single semiconductorsubstrate of single crystal silicon or other similar material usingpublicly known circuit integration techniques such as complementary MOStransistors (CMOS). The circuit symbol for Metal Oxide SemiconductorField Effect Transistor (MOSFET) without an encircled gate symbolrepresents an n-MOSFET (NMOS), while the one with an encircled gatesymbol represents a p-MOSFET (PMOS). Hereinafter, MOSFET is abbreviatedto as MOS or a MOS transistor. However, application of this invention isnot limited to field effect transistors which have an oxide insulationfilm between a metal gate and a semiconductor layer; it can be appliedto circuits which use ordinary field effect transistors like MetalInsulator Semiconductor Field Effect Transistors (MISFETs).

Although the meanings of latches, flip-flops and registers are differentin a strict sense, they are collectively referred to as latches hereinunless otherwise specified.

Embodiment 1

FIG. 1 shows a typical preferred embodiment of this invention. Accordingto this invention, the memory unit consists of a dynamic memory 100 anda cache memory 110. In the cache memory 110, numeral 111 denotes a validbit, 112 and 113 denote the address and data for each entry,respectively. Numeral 114 denotes a bus connected with the cache memory110, 115 abus connected with the dynamic memory 100, and 116 a buscontroller which controls the buses.

The dynamic memory 100 operates as illustrated in FIGS. 3A-3B. Forreading, after word line WL is asserted, the sense amplifier isactivated at the timing ΦA. As a result, when tRAC has elapsed afteraddress input, data DO is outputted. Unlike conventional dynamicmemories, rewriting operation is not performed to amplify the readsignal and send it onto the bit line and write it into a memory cell.

This means that it is unnecessary to amplify data and send it onto bitline BL, /BL as in the conventional method and thus the power requiredto charge or discharge the bit lines can be saved. Also, time whichcorresponds to tRAS shown in FIG. 2, needed in the conventional method,is not required. Although tRP is needed as a precharge time for the bitlines and so on, the required precharge time is relatively short becausethe amplitude of bit line BL, /BL remains small.

For writing, only the word line WL for the selected memory cell isasserted; as soon as the word line WL is asserted, bit line BL, /BL isactivated according to the write data.

Since rewriting into memory cells does not take place during readingoperation, this is destructive readout. The cache memory 110 is used toprotect the data. The data read from the dynamic memory 100 is sent tothe cache memory 110, which then stores the read data in a certain entrywhile the valid bit for the entry is set. In replace operation of thecache memory, regarding the entry whose valid bit has been set, as soonas new data is stored in the entry, the old stored data is written backto the dynamic memory 100 (this control is done as in the write-backprocess which uses the write process in the write allocate method).

As a result of the above-mentioned control, the data destructively readfrom the dynamic memory 100 is stored in an entry in the cache memory110; when the data is forced out of the cache memory 110 or replaced, itis written back to the dynamic memory 100 because the valid bit for ithas been set. In this way, data shuttles between the dynamic memory 100and the cache memory 110, so no original data is lost.

Data flows between the dynamic memory 100 and the cache memory 110 arecontrolled by the bus controller 116. However, as a matter of course, ifthe dynamic memory 100 and the cache memory 110 can be directlyconnected by a single bus, the bus controller as shown in FIG. 1 is notneeded.

One example of a sense amplifier suitable for the dynamic memory 100according to this invention is a sense amplifier based on the directsensing scheme as introduced on page 165 of the book about VLSI memoriesauthored by Kiyoo Itoh (published by Baifukan). In this scheme, memorycell signals can be taken out to the shared data output line withoutwaiting for the sense amplifier to amplify and send data onto the bitline, which permits quicker operation. If this scheme is directly usedin a conventional dynamic memory, an amplifier for rewriting into memorycells must be installed in parallel with the sense amplifier; on theother hand, the dynamic memory according to this invention does not needany amplifier for rewriting.

FIG. 4 shows an embodiment in which a sense amplifier based on thedirect sensing scheme is mounted in the dynamic memory 100 according tothis invention. MC represents a dynamic memory cell, numeral 301 anequalizer circuit, 302 a sense amplifier circuit based on the directsensing scheme, 303 a write amplifier circuit, 304 a word drivercircuit, 305A to 305D word lines, BL and /BL bit lines, EQ an equalizerstart signal, SA a sense amplifier start signal, and WA a writeamplifier start signal. RO and /RO denote output lines from the senseamplifier circuit, WI and /WI input lines to the write amplifiercircuit, with these two dual rail signal lines constituting I/O lines. Adistinctive feature here is the absence of a rewrite amplifier circuit.In this example, the output lines and input lines are separate lines;however, a pair of common lines may be used for both input and outputinstead. In other words, input/output lines may be either two pairs oflines (one for writing and one for reading) or one pair for both writingand reading.

As explained above, in the dynamic memory 100 according to thisinvention, tRC is remarkably shorter than in conventional dynamicmemories. When the dynamic memory 100 is pipelined as shown in FIG. 5,the pipeline pitch can be decreased by taking full advantage of thisfeature. In FIG. 5A, 200 represents the structure of a pipelined dynamicmemory according to this invention. Numeral 201 represents an addresslatch, 202, an address decoder, 203 an address driver, 204 a senseamplifier and write amplifier, 205 an input data DI latch, 206 a writebuffer, 207 an I/O line amplifier which amplifies the signals on I/Olines 210 and 211, 208 and 209 a bit line pair BL and /BL, 210 and 211an I/O line pair, 212 a word line, and 213 a memory cell. This is a2-stage pipeline structure in which clock CLK is inputted to 201, 205and 207.

For reading, the address latched by 201 is decoded and then one of theword lines 212 is selected and asserted. The memory cell data outputtedto the bit line BL, /BL is amplified by 204. The amplified memory celldata is latched by 207 according to the next clock and outputted asoutput data DO.

FIG. 5B is a timing chart for the above reading sequence. Read addressRA1 is inputted at the leading edge marked #1 of clock CLK and data RD1is outputted at #2. The data RD1 is fixed at #3, so the device orcircuit which has issued a read request at #1 can read data from thedynamic memory 100 with a latency of 2. Likewise, data RD2 whichcorresponds to the read address RA2 inputted at #2 is outputted at #3and can be read at #4. Data can be loaded 2 clocks after issuance of aread request, which means that the read latency is 2.

For writing, the address latched by 201 is decoded and then one of theword lines 212 is selected and asserted. At the same time, the writedata is latched by 205 and the bit line BL, /BL is activated by 206.This action initiates writing into a memory cell.

FIG. 5C is a timing chart for the above writing sequence. Write addressWA1 is inputted at the leading edge marked #1 of clock CLK andsimultaneously write data WD1 is inputted at #1. Writing operation iscompleted before the next clock #2, and the next write address WA2 andwrite data WD2 are inputted at #2. In response to a write request,writing can be finished within the same clock as for address input,which means that the write latency is 0.

In the above explanation of the two sequences, precharge operation forthe bit line BL, /BL, I/O line and the like is omitted. The procedurefor precharging them is not limited; precharging may be done during theperiod between the leading edge of clock CLK and the timing of word lineassertion.

One of the drawbacks of conventional dynamic memories is that when theyare pipelined, the pipeline pitch is long. A typical traditionalapproach to hide this drawback superficially is the adoption of a methodsuch as multi-bank interleave. However, such a method has the followingproblem: if the same bank is accessed successively, the pipeline mightbe disturbed and bank control is complicated.

FIG. 6A shows an embodiment in which the write latency and the readlatency are identical in the dynamic memory shown in FIG. 5. An exactdefinition of latencies as used in this application is as follows. Aread latency is the number of clocks (pulses) from the clock edge atwhich a read request is made, until the clock edge at which the data isloaded. A write latency is the number of clocks from the clock edge atwhich a write request is made, until the clock edge at which write datais inputted.

Numeral 221 represents a read address latch, 222, 223 and 224 writeaddress latches, and 225 a selector. The arrowed broken lines expressclock lines, which are controlled by a write data controller 226 asfollows. Here, the address latch shown in FIG. 5 is replaced by the readaddress latch 221, write address latches 222-224 and the selector 225.The input clocks for the address latches and the input clock for 205 arecontrolled by the write data controller 226 as follows.

As a write address is inputted, the write address latches 222-224 delaythe address. The write data which is inputted two clocks after input ofthe write address is latched by 205 and gets ready for writing. At thetiming when a next write access request is issued after this writeaccess, writing into the memory cell according to the address latched by224 and the data latched by 205 takes place with a write latency of 0.Therefore, writing is performed at the timing of the next or subsequentwrite access (actually, writing into the memory cell takes place at orafter the timing when the write address and write data both becomeavailable, or at the timing of a subsequent write request, which meansthat writing is “delayed”). While the write latency is 0 and the readlatency is 2 in the structure shown in FIG. 2, the write and readlatencies may be both 2 in the structure shown in FIG. 6A.

FIG. 6B is a timing chart for the above reading sequence. The readingsequence is basically the same as that shown in FIG. 5B. Read addressRA1 is inputted at the leading edge marked #1 of clock CLK and data RD1is outputted at #2. The data RD1 is fixed at #3, so the circuit ordevice which has issued a read request RA1 at #1 can read correspondingdata RD1 with a latency of 2. Likewise, data RD2 which corresponds tothe read address RA2 inputted at #2 is outputted at #3 and can be readat #4. Data can be loaded 2 clocks after issuance of a read request,which means that the read latency is 2.

FIG. 6C is a timing chart for the above writing sequence. Write addressWA1 is inputted at the leading edge marked #1 of clock CLK and latchedby write address latch 222. It is latched by write address latch 223 at#2 and by write address latch 224 at #3. At #3, also write data WD1 islatched by input data DI latch 205 and gets ready for writing. writingof WA1 and WD1 into the memory cell is performed at #3 or subsequentwrite input. Write data is loaded two clocks after write address input,which means that the write latency is 2.

Obviously, to achieve a write latency of 2 as shown in FIG. 6C, assumingthat a read request is issued at #3 in FIG. 6C, the input data DI latch205 should be able to latch up to 2 write data. Such a latch can beeasily realized by a first-in-first-out buffer or something like thatthough not limited so.

By making the write and read latencies identical, plural access requestsor refresh requests from the CPU or bus master can be sent to thedynamic memory without disturbing the pipeline. In addition, for adevice or circuit which uses a dynamic memory according to thisinvention, not only the read latency but also the write latency arefully predictable. Therefore, write data can be easily put into thedynamic memory with the same latency as the read latency, therebyincreasing the pipeline fill rate in case where both read and write dataare present. Especially when the output data line DO and input data lineDI are used as common input/output data lines, the above-mentionedeffect is more significant since timesharing must be used to transmitinput data and output data separately. Also, for “read modify writeaccess,” since data which has been read must be used for processingbefore writing it, the pipeline fill rate can be increased more easilywhen the read latency and write latency are the same.

In the sequence as shown in FIG. 6C, data is actually written only atleast 2 clocks after write access. Therefore, if, after write accessrequest, the same write-requested address is requested for read access,attention should be paid to data coherency. There is a solution to thisproblem as given below.

(1) If, one clock after write access request WA1, read access requestRA2 is made to the same address, write data WD1 which responds to writeaccess request WA1 must be outputted as read data RD2 which responds toread access request RA2. However, because write data WD1 has not beenwritten into a dynamic memory cell yet, it is possible that at the nextclock after read access request RA2, write data WD1 is inputted, andthen at the clock after the next clock, write data WD1 is forwarded foroutput as read data RD2 which responds to read access request RA2.

(2) If, two clocks after write access request WA1, read access requestRA2 is made to the same address, write data WD1 which responds to thewrite access request WA1 inputted at that clock may be, at the nextclock after read access request RA2, forwarded for output as read dataRD2 which responds to read access request RA2.

FIG. 7 shows another example in which a forward circuit is added to thecircuitry shown in FIG. 6A. Numeral 231 represents an addresscomparator, 232 a selector and 233 a latch. The address comparator 231compares the address data latched by write address latches 222-224 withthe read-requested address, and if an access request is made to theaddress for which writing into the memory cell has not been completed,read data which responds to the request is forwarded from the input dataDI latch 205 to the latch 223 through the selector 232.

As far as the above-mentioned operational sequences can be realized, anystructure other than the one shown in FIG. 7 is also acceptable.

Regarding the number of pipeline stages and the method of pipelinepartitioning in the dynamic memory shown in FIGS. 5 to 7 according tothis invention, any alternatives to those shown in these figures may beused. For instance, it is acceptable that a latch is provided betweenthe word decoder 202 and the word driver 203 to increase the number ofpipeline stages, or that the sense amplifier 204 is used as a pipelinelatch to increase the number of pipeline stages. Clearly, as the numberof pipeline stages increases, the pipeline pitch can be shortened,leading to a higher operating frequency.

Since the dynamic memory in this invention is used for destructivereadout, basically it is necessary to assert only the word lineconnected to the memory cell which stores the data to be read. If theword line should be asserted without data reading, the content of thememory cell would be destroyed by the word line and not read out fromthe dynamic memory so it would be lost from the entire system which usesthis dynamic memory. For this reason, when the bit width of the data tobe read at a time is small and the number of memory cells to be selectedis small, word lines should be divided into many sub-word lines and onlythe word line connected with the memory cell storing the data to be readshould be decoded and asserted. (Hereinafter, this is called the issueof word line division.) The division of word lines will result in anarea increase. However, by increasing the number of memory cells whichare read at a time, the number of sub-word lines can be decreased in amanner to prevent an area increase. This can be achieved by theprocedure mentioned below.

(1) When the cache memory 110 and the dynamic memory 100 are integratedon a semiconductor chip, the cache memory's line size can be increasedto increase the number of memory cells selectable at a time due to theabsence of a bottleneck associated with the number of pins in thepackage which contains the dynamic memory. As an extreme example, thememory cells in the cache memory may be arranged in parallel with thesense amplifier. It is also possible that the width of data between thedynamic memory 100 and the cache memory 110 is increased (for example,to 1024 bits) and the width of data between the semiconductor chipbearing them and the outside is smaller than that (for example, 32bits). By adopting these methods, the problem of a bottleneck associatedwith the number of pins in the package can be avoided and an areaincrease in the dynamic memory 100 as mentioned above can be minimized.

(2) When the primary or secondary cache of the CPU is used as the cachememory 110 and integrated on a chip other than the one bearing thedynamic memory 100, the size of data transmission only between the cachememory 110 and the dynamic memory 100 should be increased. If thesecondary cache of the CPU is used as the cache memory 110, the linesize of the secondary cache should be increased.

The data stored in the dynamic memory according to this invention ispresent in the cache memory 110 or the dynamic memory 100. If more thanone bus master are used for these memory systems, the problem ofso-called “coherency” may arise. This problem can be resolved asfollows.

(1) If the cache memory 110 and the dynamic memory 100 are integrated ona semiconductor chip and access to the chip is made only through thecache memory 110, the problem of coherency never occurs as there is nodirect access to the dynamic memory 100.

(2) If the cache memory and the dynamic memory 100 are integrated ondifferent chips, the primary or secondary cache of the CPU may be usedas the cache memory 110. Although direct access to the dynamic memory100 from more than one CPU is possible, coherency compensation such asthe snooping function using the MESI protocol, etc. incorporated in theCPU, primary cache or secondary cache controller can be used directly.When data is read from the dynamic memory 100, the valid bit for theentry of the data is set and thus the MESI protocol monitors access tothe entry from other CPUs.

FIGS. 8A and 8B shows a fully pipelined dynamic memory as an embodimentof this invention, where the cache memory 110 cannot be used. Since thedynamic memory according to this invention is of the destructive readouttype as mentioned earlier, data once read is not present in the dynamicmemory. FIGS. 8A and 8B indicate that in the pipelined dynamic memory,immediately after reading (RA1, RD1), writing (WA1, WD1) of the readdata in the same address takes place. FIG. 8A shows waveforms for theexample shown in FIG. 5. FIG. 8B shows waveforms for the example shownin FIG. 6 or FIG. 7 which uses the delayed write scheme. As describedabove, when the scheme shown in FIG. 6 or FIG. 7 is used, a new accessrequest (RA2) can be accepted at #3, so access overhead can be reducedto only 1 clock. If more than one bus master are provided, write accessin continuous read/write operation for rewriting as mentioned aboveshould be most preferred for coherency compensation. (Hereinafter, thedata retention method for destructive read memory cells which uses thistype of pipeline is called a “pipeline rewrite technique.”)

The methods illustrated in FIGS. 8A and 8B can be used not only when thecache memory 110 is not available but also when valid bit control forthe cache memory 110 is impossible. It can also be used when the cachememory 110 is an instruction cache.

The cache memory 110 shown in FIG. 1 may be integrated on the samesemiconductor chip that bears the dynamic memory 100, or may beintegrated on a different chip.

When the dynamic memory 100 is used as the main memory of the CPU, it isbest to use the cache memory 110 as the primary cache of the CPU.Alternatively, it is acceptable to construct a memory system consistingof the primary and secondary caches of the CPU. In this case, theoptimum sequence is as follows: the data read from the dynamic memory100 is written in the primary cache; and when the data is deleted fromthe primary cache, it is written in the secondary cache; then when it isreplaced in the secondary cache, it is written back into the dynamicmemory 100. As stated above, the cache memory 110 may also serve as theprimary or secondary cache of the CPU to enhance the area efficiency.

The number of cache memories 110 is not limited. It is also possiblethat the cache memory 110 has more than one hierarchical memory level.Two caches—one instruction cache and one data cache—may be provided. Forthe data cache, the access process based on the valid bit as explainedfor the example in FIG. 1 may be used, while for the instruction cache,the process described for the example in FIG. 8 may be used for writeaccess to be made after read access. Another approach is that thedynamic memory 100 provides two modes: one mode for dynamic memoryaccess according to this invention and the other mode for conventionaldynamic memory access. Depending on the type of access, the mode formore efficient access may be selected, which permits more efficient useof the dynamic memory 100.

Though the above examples use valid bits, the existence of valid bits isnot a prerequisite. Also, the line size, the number of ways, capacityand so on of the cache memory 110 are not limited. It is sufficient thatthe data destructively read from the dynamic memory 100 is stored in thecache memory 110, and the data forced out of the cache memory 110 isstored in the dynamic memory 100. If two or more cache memories areprovided, control should be made in a way that data is always present inthe cache memories and the dynamic memory. The important thing is thatthe data destructively read from a dynamic memory should be stored inany memory (cache memory in this invention) available throughout thesystem which uses the dynamic memory, except the dynamic memory. As faras this condition is met, any system configuration may be used.

Furthermore, the number of dynamic memories 100 is not limited. Themethod disclosed in this invention may apply to plural dynamic memorychips or to some of plural dynamic memory chips.

Besides, the structure of memory cells in the cache memory 110 is alsonot limited. It may be of the dynamic type in which a capacitance storeselectric charge to memorize data, or of the SRAM memory cell type whichuses poly-resistors or TFTs, or fully complementary MOS SRAM type whichuses six MOS transistors.

Embodiment 1 of this invention explained so far is summarized asfollows.

(1) In a semiconductor device which has a dynamic memory which hasplural dynamic memory cells provided at intersections of plural wordlines and plural bit lines, and plural sense amplifiers provided for therespective plural bit lines, and plural input/output lines provided forthe respective plural sense amplifiers, for reading, the dynamic memoryselects one of the word lines and reads out the signal of thecorresponding dynamic memory cell to the corresponding plural bit lines;then without transition to the phase of rewriting of the read signalinto the dynamic memory cell, the plural sense amplifiers amplify thesignal read out to the bit lines, on the input/output lines before theplural bit lines are precharged (first read mode 1).

(2) In addition, the dynamic memory further has a write amplifier forthe corresponding bit line, and for writing into the dynamic memorycell, the write amplifier outputs write signal to the corresponding bitline, just after or before or at the same time as selection of thecorresponding word line, to write the signal into the dynamic memorycell (first write mode).

(3) The semiconductor device as described above in (1) and (2) isfurther provided with at least one cache which consists of static memorycells, and in reading data from the dynamic memory, the reading sequenceas mentioned above is used to read data from the dynamic memory and thedata is written in at least one the cache, and when the data is deletedfrom all the caches, the data is written back to the dynamic memory.

The semiconductor device which includes a dynamic memory as describedabove in (1) through (3) is provided with an address latch circuit whichreceives the row address to select the word line to be accessed, fromamong the plural word lines, where the address latch circuit latches therow address at each transition point of the first clock signal which hasa prescribed cycle time.

(5) The pipelined dynamic memory as mentioned in (4) is further providedwith a write delay circuit which receives a first write address and afirst write data which are inputted at a first write access, wherewriting into the dynamic memory cell in response to the first writeaccess is performed for the first write address and the first write datawhich are stored in the write delay circuit, at the timing of the secondwrite access after the first write access.

(6) The dynamic memory as described in (5) is further provided with aforward circuit having an address comparator, where, in read access, theforward circuit compares the inputted read address with the first writeaddress through the address comparator, and if there is a read access tothe same address as the first write address between the first writeaccess and the second write access, the first write data is outputted asread data which responds to the read access.

Embodiment 2

FIG. 9 shows a pipelined DRAM (PDRAM) as an embodiment of this inventionin more concrete form. MO represents an n-MOS transistor and a memorycell MC1 consists of an n-MOS transistor and a capacitor CO. BL1 to BLnrepresent bit lines, WL to WLm word lines and the above-said memorycells are connected at the intersections of the word lines and bitlines. (In the folded bit-line structure as stated on page 90 of thebook about VLSI memories by Kiyoo Itoh, published by Baifukan in 1994,memory cells are not always connected at the intersections of bit linesand word lines. This invention does not limit the bit line arrangementto the one shown in FIG. 9.) RAMP denotes a read amplifier, WAMP a writeamplifier, and LX-DEC a word line decoder (including a word line drivercircuit). SARY1 to SARYx represent sub-arrays composed of theabove-mentioned circuits and the like. WDATAL denotes a write datalatch, RDATAL a read data latch, WSEL a write data selector, RSEL a readdata selector, Y-DEC a Y decoder (including a Y driver), Y-ADRL a Yaddress latch, GX-DEC a global word line decoder (row decoder includinga global word line driver), X-ADRL an X address latch (row address latchcircuit), CRL a timing control circuit, GWL1 to GWLz global word lines,DI1 to DIn input data, DO1 to DOn output data, ADD an address (anaddress is inputted without being multiplexed), WE a write enable, CLK aclock, and VPL a plate voltage.

The inputted address ADD is latched by X-ADRL and Y-ADRL at every cycleof clock CLK and decoded by GX-DEC and Y-DEC. As a result of decoding byGX-DEC, one of the global word lines GWL1 to GWLz is selected. As aresult of decoding by Y-DEC , one of the sub-arrays SARY1 to SARYx isselected. The decoding result for the global bit lines GWL1 to GWLz andY-DEC is inputted to LX-DEC and one of the word lines WL1 to WLm in theselected sub-array is selected and activated. The number of memory cellsto be selected by a word line is the same as the number of output orinput data, n.

For reading, stored data from n selected memory cells are amplified by nread amplifiers RAMP. The amplified n data DO1A to DOnA are inputted toread selector RSEL. According to the decode signal inputted from Ydecoder Y-DEC, RSEL selectively connects n data DOlA to DOnA outputtedfrom sub-arrays SARY1 to SARYx to n inputs of read data latch RDATAL.The n data sent to the read data latch RDATAL are latched by read datalatch RDATAL according to clock CLK, and outputted as DO1 to DOn tooutside the pipelined dynamic memory PDRAM.

For writing, input data DI1 to DIn are latched by write data latchWDATAL according to clock CLK, and inputted to write data selector WSEL.WSEL selects a sub-array for writing according to the decode signalinputted from Y decoder Y-DEC, and n data from WSEL are selectivelyconnected to n inputs DI1A to DInA of sub-arrays SARY1 to SARYx. Theinputted n data DI1A to DInA are amplified by write amplifier WAMP andwritten in n selected memory cells as memory data through bit lines.

In the structure shown in FIG. 9, the operational sequences shown inFIG. 3A and FIG. 3B can be achieved by pulse-driving the word lines WL1to WLm for a specified period within the clock CLK cycle. Also, asmentioned earlier, because rewriting is not made, the pulse width of theabove word lines can be shortened, enabling the pipeline pitchdetermined by it (clock CLK cycle) to be shorter. Since the structure inFIG. 9 is basically the same as that of embodiment shown in FIG. 5A,timing charts which illustrate the reading and writing sequences in thiscase are the same as those in FIGS. 5B and 5C.

In the structure shown in FIG. 9, the issue of word line division asmentioned earlier is resolved by hierarchically dividing the word linesinto global word lines GWL1 to GWLm and word lines WL1 to WLm. Thenumber of global word lines GWL1 to GWLm are the same as the number ofword lines GWL1 to GWLm, but as the number of decode address bits of theY decoder is increased, the number of global word lines GWL1 to GWLm canbe smaller than m.

Though FIG. 9 does not show WAMP and RAMP circuits concretely, it ispossible to use, for example, the circuits shown as 303 and 302 in FIG.4 for WAMP and RAMP, respectively. In FIG. 9, WAMP and RAMP are locatedat both ends of each bit line for better illustration. The actualcircuit layout is not limited to this layout. WAMP and RAMP may belocated at one end of each bit line like 303 and 302 in FIG. 4. In thatcase, it is needless to say that write data selector WSEL and read dataselector RSEL may be shared. Further, a “shared sense amplifier” systemmay be used by connecting bit lines to both ends of RAMP and WAMP andconnecting memory cells to the respective bit lines. As discussed above,the bit line arrangement and the RAMP and WAMP arrangement are notlimited to the arrangements shown in FIG. 9. Besides, again for betterillustration, no precharge circuit like 301 in FIG. 4 is not shown inFIG. 9, but obviously any circuit necessary for memory circuitoperation, such as precharge circuits, may be added.

Embodiment 3

Next, a refresh-free dynamic memory (RFPDRAM) which uses a pipelineddynamic memory as shown in FIG. 9, etc. is explained as anotherembodiment.

FIG. 10 shows RFPDRAM as an embodiment. Here, PDRAM corresponds to thepipelined dynamic memory as shown in FIG. 9, etc. ASEL, DISEL and WESELrepresent selectors, RFADDG a refresh address generator, RFDATL arefresh data latch (data latch circuit), REFSEQ a refresh sequencer andFF1 a flip-flop, all of which constitute an access control circuitACCRL. Here, FF1, which is what is generally called a flip-flop,memorizes input D at the timing of transition from “L” to “H” of theclock inputted at the clock input marked with a triangle, and outputs itthrough Q, while under other conditions the output at Q remainsunchanged.

ADD represents an address terminal for PDRAM, DI a data input terminalfor PDRAM and DO a data output terminal for PDRAM, and they each have aspecified number of bits depending on the PDRAM capacity and the numbersof input and output bits. On the other hand, EADD represents an addressterminal for RFPDRAM, EDI a data input terminal for RFPDRAM and EDO adata output terminal for RFPDRAM, and they each have as many bits asADD, DI and DO for PDRAM. WE and EWE denote write enable signals forPDRAM and RFPDRAM, respectively. CLK, CLK1 and CLK2 represent clocksignals or clock terminals.

The address EADD inputted to the refresh-free dynamic memory RFPDRAM issent to selector ASEL together with output RFADD of refresh addressgenerator RFADDG and selectively connected to address ADD of PDRAMaccording to the value of selector signal P1. Similarly, the input dataDI inputted to RFPDRAM is sent to selector DISEL together with outputRFDAT of refresh data latch RFDATL, and selectively connected to inputdata DI of PDRAM according to the value of selector signal P1. The writeenable signal EWE inputted to RFPDRAM is sent to selector WESEL togetherwith output RFWE of refresh sequencer REFSEQ and selectively connectedto write enable signal WE of PDRAM according to the value of selectorsignal P1. Output data DO of PDRAM is sent to refresh data latch RFDATLtogether with output data EDO of RFPDRAM. REFSEQ carries out necessarycontrol for refreshing PDRAM by using clock CLK1 and CLK2 inputted toRFPDRAM to control refresh address generator RFADDG, refresh data latchRFDATL and P1. FIG. 11 is a timing chart for an operation example.

Clock CLK1 is a clock whose frequency is twice that of clock CLK2, andtheir leading edges or rise timings coincide. External access requestsfrom a device or circuit which uses RFPDRAM connected with EADD, EDI,EWE or the like are loaded at the timing of rise of CLK2. (For simplerillustration, such a device or circuit is not shown in FIG. 10 (?).Hereinafter it is called an external device and an access request fromthe external device to RFPDRAM is called an external access request.)Since output P1 in FF1 in FIG. 10 is “H” at the timing of rise of CLK2,the access request loaded at the timing of rise of CLK2 is directly sentto PDRAM for processing. In FIG. 11, as external requests, read requestRA, write request WA2, read request RA3, read request RA4, and readrequest RA5 are loaded at #1, #3, #5, #7 and #9, respectively; inresponse to these access requests, RFPDRAM makes fixed output of readdata RD1 at #3, input of write data WD2 at #3, fixed output of read dataRD3 at #7, fixed output of read data RD4 at #9, and fixed output of readdata RD5 at #11. Read data is outputted with a latency of 2 in terms ofCLK1 frequency, or a latency of 1 in terms of CLK2 frequency, or nowaiting time. Write data is outputted with a latency of 0.

As stated above, an external access request to the pipelined dynamicmemory PDRAM is generated only every two cycles. In FIG. 11, externalaccess requests occur only at the leading edges of odd-numbered clocks#1, #3, #5, #7 and so on. As PDRAM is fully pipelined, a request foraccess to PDRAM can be made every cycle of CLK1. On the other hand, incase of the structure shown in FIG. 10, an external access request canbe issued at most every two cycles of CLK1, as described above. Theaccess control circuit ACCRL issues access requests for refreshing toPDRAM in the intervals between these access requests (unoccupiedpipeline slots indicated as #4, #6, #8, #10 and so on). This sequence isdetailed next by reference to FIG. 11.

The access control circuit ACCRL issues refresh requests to PDRAM atregular time intervals so that the data stored in the pipelined dynamicmemory PDRAM is not erased. In FIG. 11, for the refresh address RFADDgenerated by the refresh address generator RFADDG, read request RA0 isissued at #4, and the corresponding read data RD0 is received at #6 andstored in the refresh data latch RFDATL. Then, at #8, write request WA0is issued to the same address to which the read request has been made,so that the dataWD0 stored in RFDATL is written there. With the abovesequence, rewriting of memory cells in PDRAM takes place. After that,RFADDG counts up RFADD. This operational sequence is repeated at regulartime intervals by the refresh sequencer REFSEQ so that all the memorycells in PDRAM can be refreshed.

In the above example, refreshing operation unique to any dynamic memoryin which the electric charge stored in the capacitance is used tomemorize data, can be completely concealed from a device or circuitwhich uses the dynamic memory. Also, in terms of access speed (latenciesin this case), it delivers the same performance as the originalpipelined dynamic memory PDRAM. (In the example in FIG. 11, the readlatency is 2 in terms of CLK1, which suggests no decline in speed.) Onthe other hand, the maximum frequency (freql) of access request whichcan be issued to RFPDRAM is half the maximum frequency (freq) of accesswhich can be accepted by PDRAM. However, as the pipeline frequency(frequency of CLK1) of PDRAM can be sufficiently high as a result ofpipelining, the speed can be increased to the extent that the frequency(freq1) of access request to RFPDRAM is negligible. For instance, incase of a microprocessor which runs at 300 MHz as an external devicewhich uses a refresh-free dynamic memory RFPDRAM, CLK may be used at 600MHz and CLK1 at 300 MHz.

FIGS. 9 to 11 show the case that the read latency and write latency ofthe pipelined dynamic memory PDRAM are 2 and 0, respectively, butnaturally, the application of the above refresh concealing technique isnot limited to the case of using these latencies. However, if PDRAM'slatency in terms of CLK1 is expressed as L and reception andtransmission of data in response to an external access request arecarried out in the cycle of CLK2, latency L1 in terms of CLK1 is L/2, anumber whose decimal fractions are counted as a whole number. Therefore,the latency in terms of CLK1 for data reception and transmission inresponse to an external access request is L+1 if L is an odd number.

Embodiment 4

Refresh concealing techniques other than the one shown in FIG. 11 may beused. In the example in FIG. 11, since the PDRAM pipeline cycle is halfthe cycle of external access requests and external access requests areissued only at #1, #3, #5, #7 and so on of pipeline clock CLK1 of thedynamic memory, chances for refreshing can be obtained at #4, #6, #8 and#10 of CLK1. In other words, the phases of external access requests aremade different from those of refresh-related access requests in order toprevent collision of both types of requests. In this way, accesscollision may be prevented by making the phases for both types of accessdifferent.

In addition, when the cycle of the PDRAM pipeline is shorter than thatof external access requests, chances to refresh PDRAM can be obtainedwithout fail even if external access requests are issued continuously.This means that it is acceptable that pipeline frequency CLK1 is notdouble that of clock signal CLK2 corresponding to the cycle forreception of external access requests, unlike the example in FIG. 11.For instance, the ratio of CLK1 frequency to CLK2 frequency is arational number above 1, so 3/2 is acceptable. In this case, even if anexternal access request is issued to RFPDRAM every cycle of CLK2, therewill be, every three cycles of CLK1, a period during which no externalaccess request is issued to PDRAM. Also, if the above-said frequencyratio is 1000/999, there will be a period during which no externalaccess request is issued to PDRAM, every 1000 cycles of CLK1. Therefore,it is sufficient for the refresh sequencer REFSEQ to issue an accessrequest for refreshing to PDRAM at the timing when no external accessrequest is present. Generally speaking, the refresh cycle is longer thanthe cycle of external access requests; so even if the frequency ratio ofCLK1 to CLK2 is as small as 1000/999 or so, the CLK1 frequency can beincreased to the extent that refreshing can be sufficiently done in acycle.

FIGS. 12 and 13 show examples in which the frequency ratio of CLK1 toCLK2 is 3/2, in more detailed form. The example shown in FIG. 12 isdifferent from that in FIG. 10 in the following two points. (1) In placeof the circuit which creates flip-flop FF1 to generate select signal P1for selectors ASEL, DISEL and WESEL in FIG. 10, a circuit composed offlip-flops FF2, FF3 and FF4 is used in FIG. 12; here P3 representsselect signal for selectors ASEL, DISEL and WESEL. (2) EADD, EDI, EDOand EWE are connected to selector ASEL, selector DISEL, DO terminal ofPDRAM, and selector WESEL through flip-flop FF5, flip-flop FF6, latchTL1 and flip-flop FF7, respectively. Here, in latch TL1, output Qfollows the data inputted to D as far as clock input E is “H.” As clockinput E becomes “L,” Q output is retained until clock input E becomes“H.” Like the example in FIG. 10, refresh sequencer REFSEQ uses clockCLK1 and CLK2 inputted to RFPDRAM to control refresh address generatorRFADDG and refresh data latch RFDATL, and P2 and P3 so that PDRAM isrefreshed adequately. FIG. 13 is a timing chart for this operationalsequence.

Clock CLK1 is a clock whose frequency is 1.5 times that of clock CLK2and their phase relationship is shown in FIG. 13. External accessrequests are loaded at the timings of rise of CLK2 through flip-flopsFF5, FF6 and FF7 in the same way as in FIG. 11. The select signal P3 forselectors ASEL, DISEL and WESEL has a waveform as shown in FIG. 13, soan external access request loaded at the timing of rise of CLK2 is putinto the pipelined dynamic memory PDRAM at rise timings of CLK1 afterthat timing. Here, read request RA1, write request WA2, read requestRA3, read request RA4, and read request RA5 are loaded as externalaccess requests at #1, #2, #3, #4 and #5 of CLK2, respectively; and readrequest RA1, write request WA2, read request RA3, read request RA4 andread request RA5 are loaded into PDRAM at #2, #3, #5, #6 and #8 of CLK1,respectively. In response to these access requests, RFPDRAM makes fixedoutput of read data RD1 at #4 of CLK1, input of write data WD2 at #3,fixed output of read data RD3 at #7, fixed output of read data RD4 at#8, and fixed output of read data RD5 at #10. Each of the read data isoutputted through latch TL1 where P2 clock shown in FIG. 13 has beeninputted, when read data RD1, read data RD3, read data RD4 and read dataRD5 are fixed for output at #3, #5, #6 and #7 of CLK2, respectively.Read data is outputted with a latency of 2 in terms of CLK2 frequency,or a latency of 3 in terms of CLK1 frequency, or no waiting time. Writedata is outputted with a latency of 0.

As stated above, an external access request to the pipelined dynamicmemory PDRAM is generated only twice every three cycles. In FIG. 13,external access requests are generated twice every three cycles only atthe leading edges of CLK1, like #2, #3, #5, #6 and so on. As PDRAM isfully pipelined, a request for access to PDRAM can be made every cycleof CLK1. On the other hand, in case of the structure shown in FIG. 12 or13, an external access request can be issued at most twice every threecycles of CLK1. The access control circuit ACCRL issues access requestsfor refreshing to PDRAM in the intervals between these access requests(unoccupied pipeline slots indicated as #4, #7 and so on of CLK1). Thissequence is detailed next by reference to FIG. 13.

To prevent the data stored in the pipelined dynamic memory PDRAM frombeing erased, the access control circuit ACCRL issues refresh requeststo PDRAM at regular time intervals. In FIG. 13, for the refresh addressgenerated by the refresh address generator RFADDG, read request RA0 isissued at #4 of CLK1, and the corresponding read data RD0 is received at#6 of CLK1 and stored in the refresh data latch RFDATL. Then, at #7 ofCLK1, write request WA0 is issued to the same address to which the readrequest has been made, so that the dataWD0 stored in refresh data latchRFDATL is written there. With the above sequence, rewriting of memorycells in PDRAM takes place. After that, refresh address generator RFADDGcounts up the refresh address RFADD. This operational sequence isrepeated at regular time intervals by the refresh sequencer REFSEQ sothat all the memory cells in PDRAM are refreshed.

Like FIGS. 11 and 12, in the examples shown in FIGS. 12 and 13,refreshing operation unique to any dynamic memory in which the electriccharge stored in the capacitance is used to memorize data, can becompletely concealed from a device or circuit which uses the dynamicmemory. Also, in terms of access speed (latencies in this case), theread latency is 3 in terms of CLK1 or 2 in terms of CLK2. The readlatency is larger than PDRAM's original latency only by 1, so thatrefreshing operation can be completely concealed.

Alternatively, thanks to the feature that PDRAM is fully pipelined, readand write requests for refreshing can be repeatedly made in theintervals between external access requests. Other various refresh meanswhich take advantage of the feature that PDRAM is fully pipelined arepossible without disturbing external access requests. Combinations ofdifferent refresh means are acceptable. When an external access requestand a refresh request collide, if the external access request may bedelayed, it is apparent that more variations of refresh means arepossible.

Embodiment 5

In the examples of refresh dynamic memory RFPDRAM shown in FIGS. 10 and12, clocks CLK1 and CLK2 are inputted from outside; however, other clocksources maybe used. CLK2 may be generated from CLK1 by means of a clockdivider or the like, or CLK1 may be generated from CLK2 by means of aclock doubler such as PLL (phase locked loop). FIG. 14 shows an examplewhich has a PLL-based clock distribution in addition to the structure inFIG. 10. CLKGEN represents a clock generating circuit; in this case itis a clock doubler with a PLL structure. CLKSYS represents a clockdistribution inside PDRAM; in this case it is an H-tree clockdistribution though not limited so. In the figure, the triangles (e.g.406) express clock buffers, which distribute CLK1 to the latch circuits405A to 405G which use CLK1 with no skew (here circuits which uselatches, flip-flops or clocks such as selectors are collectivelyreferred to as latch circuits). CLK1 is also distributed to clockgenerating means 403 as CLK1A at the same timing (phase) as when it isdistributed to latch circuits 405A to 405G. Besides, clock CLK2 is alsosupplied to the clock generating circuit CLKGEN. Since the clockgenerating circuit CLKGEN has a PLL structure, it includes a phasecomparator circuit so that CLK1 is generated in a way that the followingconditions are met: (1) The phases of CLK1A and CLK2A are the same; (2)The frequency of CLK1A is double that of CLK2.

As mentioned above, when, in PDRAM, CLK1 which is distributed using theclock distribution with no skew is fed back to the clock generatingcircuit as CLK1A with no skew, the phase of CLK2 supplied to RFPDRAM isthe same as the phase of CLK1 which is received by latch circuits 405Ato 405G. This makes it easy to obtain setup margins or hold margins forvarious signals such as ADD, EADD, DO, EDO, DI, EDI, WE and EWE, soPDRAM can be operated at higher frequencies. Particularly, the effect ofthe above method is significant for PDRAM whose area is large, because aconsiderable delay occurs between the clock at the output point of theclock generating circuit CLKGEN and the clock received by latch circuits405A to 405AG.

The clock generating circuit shown in FIG. 14 may have any structureother than PLL. It may have a structure such as DLL (delay docked loop)or SMD (synchronous mirror delay). It may have any structure as far asclocks with desired frequencies can be generated by making the phases oftwo inputted clocks coincide. cl Embodiment 6

The refreshing process in the refresh concealing technique used inembodiments 3 and 4 can also be used for access to PDRAM for purposesother than refreshing. For example, it can be used for rewrite accesswith the pipeline rewrite technique shown in FIG. 8. Specifically, PDRAMmay be used in a manner that the frequency of external access is smallerthan the pipeline frequency which depends on the performance of PDRAM sothat remaining time can be used for rewriting as mentioned above. Thus,a fully pipelined high-speed dynamic memory which uses destructive readmemory cells can be realized without using the cache 110.

The number of pipeline stages and the pipeline partitioning method arenot limited to those shown in FIG. 9 and those shown later in FIG. 15.One method of increasing the number of pipeline stages is to input clockCLK to word line decoder LX-DEC to add a latch function; another methodis to use read amplifier RAMP or write amplifier WAMP as a pipelinelatch. Needless to say, as the number of pipeline stages is increased,the pipeline pitch can be decreased to increase the operating frequency.

In the above-said examples of pipeline dynamic memory PDRAM, output datafrom memory cells are outputted to the outside of PDRAM through latches(hereinafter called output latches). In the example shown in FIG. 9,read data latch RDATL is provided as an output latch. However, therefresh concealing technique used in this invention may be embodiedregardless of the presence of this output latch. It can be used forflow-through type synchronous dynamic memories. Naturally the latencyvaries according as whether there is an output latch or not.

It is also possible to add a delayed write function as used in FIG. 6 orFIG. 7 to the example in FIG. 9; even if that is the case, obviously therefresh concealing technique shown in FIGS. 10 to 14 according to thisinvention can be realized only by using a small circuit. By making thewrite latency and the read latency equal, plural access requests orrefresh requests from plural bus masters of CPUs or the like can be fedto the dynamic memory without disturbing the pipeline. Unlike theexample in FIG. 9 where output data lines DO and input data lines DI areseparate lines, if input/output data lines serve as both output datalines DO and input data lines DI, input data and output data must beseparated for timesharing; in such a situation, the delayed writefunction is very effective. In so-called “read modify write access,”after processing is done using the data which has been read, the datamust be written; in such a situation, when the read latency is equal tothe write latency, a higher pipeline fill rate is achievable.

Obviously, even if it is not exactly the same type of pipelined dynamicmemory PDRAM as illustrated in FIG. 9 etc, the above-said refreshconcealing technique can be applied as far as it is a pipelined dynamicmemory. PDRAM need not be of the non-rewritable type as shown in FIG. 3.For rewriting, the word line assert time is relatively long as shown inFIG. 2, and thus the pipeline pitch must be longer, which makes itdifficult to increase the pipeline frequency. However, the refresh datalatch RFDATL in FIGS. 10 and 12 is not needed and the refresh sequencerREFSEQ has only to issue read access requests to addresses generated bythe refresh address generator RFADDG.

Although the refresh concealing technique mentioned above uses pipelineoperation, it can be used even in a non-pipelined dynamic memory, forinstance, a synchronous dynamic memory (SDRAM). (Though even SDRAMs arepipelined with regard to column access, the term “pipelining” used inthis application means pipelining associated with row access.)Concretely, when the external cycle time is set to a value larger thanthe cycle time which depends on the circuit performance, remaining timecan be used for refreshing. For instance, it is a good idea to makeSDRAM's external access request cycle time double that of the accesscycle time which can be basically executed by SDRAM. This can beachieved by making the time interval (tRC=tRAS+tRP) between bank activecommands to the same bank double the level executable by the circuit.This means that if that time interval (2×tRC) is used, prechargecommands can be executed by two bank active commands. One of theprecharge commands by the two bank active commands should be used toprocess an external access request, while the other precharge commandshould be used for refreshing operation as necessary. Due to thenon-pipelined structure, the cycle time doubles, and the latency alsovirtually doubles if a delay in external access which occurs duringrefreshing operation as mentioned above is included. This type ofdynamic memory is relatively low in performance; however, because it cancompletely conceal refreshing operation, it performs external controleasily and thus is easy to handle.

Embodiment 7

In the example shown in FIG. 9, memory cells which each consist of ann-MOS transistor and a capacitor are used to memorize data. In short, itis assumed to use destructive read memory cells (hereinafter called 1Tmemory cells) in which the data inside them is destroyed as it is readout. The memory cells used in this invention are not limited to thistype. The invention can be applied to dynamic memories which usenon-destructive read memory cells (hereinafter called 3T memory cells)each consisting of three n-MOS transistors as stated on pages 42-43 of1970 IEEE International Solid-State Circuits Conference Digest ofTechnical Papers. Many other variations of dynamic memory cells such asones which use four MOS transistors may be used. In case of using 3Tmemory cells, read word lines and write word lines may be eitherseparate lines or common lines, or read bit lines and write bit linesmay be either separate lines or common lines. The structure and controlmethod of these memory cells are also not limited.

FIG. 15 shows an example of a pipelined dynamic memory PDRAM which uses3T memory cells. MC2 represents a 3T memory cell. Data is memorized bythe electric charge stored in the gate terminal of n-MOS transistor M2.Word lines WL1 to WLm and global word lines GWL1 to GWLz are controlledaccording to ternary data. At medium potential, the current whichdepends on the potential of the gate of n-MOS transistor M2 is fed tobit line RBL through n-MOS transistor M1 for reading. For writing, highvoltage is applied to word line WL to turn on n-MOS transistor M3, andvoltage from bit line WBL is directly applied to the gate potential ofn-MOS transistor M2.

The example in FIG. 15 is different from that in FIG. 9 in the followingtwo points. One point is that while in FIG. 9 the memory cells are 1Tmemory cells, in FIG. 15 the memory cells are 3T memory cells and,therefore, two types of bit lines—read bit lines RBL1 to RBLX and writebit lines WBL1 to WBLx—are used. The second point is as follows. In FIG.9, write data is amplified by write amplifier WAMP after passing throughwrite data selector WSEL, and then sent to the bit lines, and the readdata on the bit lines is amplified by a read amplifier before beingoutputted through read data selector RSEL. On the other hand, in FIG.15, write data is amplified by write amplifier WAMP before being sent tothe bit lines through write data selector WSEL, the read data on the bitlines is passed through read data selector RSEL and then amplifiedbyread amplifierbefore being outputted. Thus, one read amplifier RAMP orwrite amplifier WAMP is shared by more than one Y address. The readamplifier RAMP and the write amplifier WAMP are shared by plural bitlines as shown in FIG. 15. This sharing of the read amplifier RAMP orthe write amplifier WAMP offers an advantage that the area available foreach amplifier can be larger than when they are not shared. A largerarea for each amplifier gives more latitude of choice of amplifiertypes, which implies that the use of a higher speed amplifier such as acurrent sense amplifier is possible.

If 3T memory cells which have read word lines and write word lines like10(a) in FIG. 1 on page 13 of the book about VLSI memories by Kiyoo Itoh(published by Baifukan in 1994) are used, the hierarchical arrangementof read word lines as shown in FIG. 15 is no longer needed. This isbecause 3T memory cells are non-destructive read cells and thus it isallowed that memory cells are not read out even after word lineassertion.

Like FIG. 9, FIG. 15 does not illustrate any concrete circuits for WAMPand RAMP. The arrangement of RAMP, WAMP and bit lines is not limited tothat shown in FIG. 15. In FIG. 15, WAMP and RAMP are located at bothends of the bit lines in order to make the illustration easy tounderstand. The actual circuit layout is not limited to this layout.WAMP and RAMP may be located at one end of the bit lines like 303 and302 in FIG. 4. In that case, it is needless to say that write dataselector WSEL and read data selector RSEL may be shared depending on thelayout. Further, a so-called “shared sense amplifier” system may be usedby connecting bit lines to both ends of RAMP and WAMP and connectingmemory cells to the respective bit lines. Besides, no precharge circuitlike 301 in FIG. 4 is not shown in FIG. 15 for purposes of betterillustration, but obviously any circuits necessary for memory circuitoperation, such as precharge circuits, may be added in adequate places.

Even when non-destructive read memory cells like the 3T memory cellsshown in FIG. 15 are used in the pipelined dynamic memory PDRAMaccording to this invention, tRAS shown in FIG. 3A is not needed andthus a short reading operation as expressed by tRP can be achieved. Thisoffers an advantage that the cache memory 110 is not needed. Evidently,with 3T memory cells, pipeline operation according to this invention, asshown in FIGS. 5 to 14, can be performed in the same way as when 1Tmemory cells are used.

Depending on the supply voltage, if a capacitor which complicates theprocess is not added to the storage node of a 3T memory cell (gateterminal of n-MOS transistor M2), the retention time is shorter thanwhen 1T memory cells are used. Also, since rewriting is not done duringreading, the actual retention time is shortened. In addition to theabove factors, 3T memory cells which use logic processes have morefactors which shorten the retention time, than 1T memory cells. As theretention time decreases, the frequency of refreshing increases.However, this disadvantage can be compensated for by using theaforementioned refresh concealing technique according to this inventionor a similar technique to reduce refresh-related overhead.

FIG. 16 shows an example of a merged DRAM/logic LSI (EMCHP) whichincorporates a refresh-free dynamic memory RFPDRAM according to thisinvention. Here, 3T memory cells as shown in FIG. 15 are used as memorycells. Regarding MOS symbols used in FIG. 16, a part with a gateelectrode as expressed by a box with a white inside area (e.g. M512)denotes a high voltage MOS transistor which has a larger gate-oxidethickness (e.g. 6.5 nm), while a part with a gate electrode as expressedby a line (e.g. M522) denotes a MOS transistor which has a smallergate-oxide thickness (e.g. 3.2 nm).

VDD and VSS represent a core power supply and its earth, VDDQ and VSSQrepresent an I/O power supply and its earth. For instance, the corepower supply voltage is 1.0 V and the I/O power supply voltage is 3.3 V.OUT0 to OUTx represent output signals, IN0 to INy input signals, I/O0 toI/Oz input/output signals. PADCB shows an I/O circuit for interfacingchip inside signals with the outside of the chip, and 511 shows a finaldriver circuit which is composed of a p-MOS transistor M512 and an n-MOStransistor M513 which have a thick gate-oxide thickness. Numeral 514represents an initial buffer circuit which is composed of a p-MOStransistor M515 and an n-MOS transistor M516 which have a thickgate-oxide thickness. (It is advisable to use MOS transistors with athick gate-oxide thickness as MOS transistors in an ESD device toprevent electrostatic destruction, though such a device is not shownhere to illustrate 514 in a simplified form.) LCB shows a logic circuitwhich incorporates inverters and NAND gates. FIG. 16 shows an invertercircuit 521 which incorporates p-MOS transistor M522 and n-MOStransistor M523 which have a thin gate-oxide thickness. Examples of LCBare a logic circuit with more than 10,000 gates like a microprocessor orDSP, and SRAM. The 3T memory cells in RFPDRAM use the same MOStransistors as the thick-gate-oxide MOS transistors used in the I/Ocircuits. (Since high voltage maybe applied ton-MOS transistors M1 andM3 in MC in FIG. 15, the transistors should be thick-gate-oxide MOSs.However, since high voltage is not applied to M2, thin-gate-oxide MOStransistors may be used depending on the process and memory cell size.)

In the example shown in FIG. 16, MOS transistors in which high voltagemight be applied between the gate and source electrodes or between thegate and drain electrodes are thick-gate-oxide MOSs, and other MOStransistors are thin-gate-oxide MOSs for higher speed processing. If,like the example in FIG. 6, only two types of gate oxide thickness areused throughout the chip, the fabrication process can be simplified.

Generally speaking, if a dynamic memory based on 1T memory cells and alogic LSI are integrated on a chip, the chip fabrication process wouldbe more complicated. However, if a dynamic memory uses 3T memory cells,memory cells need not incorporate a capacitor; therefore, thefabrication process would be less complicated than when 1T memory cellsare used. In addition, if the arrangement shown in FIG. 16 is used, thesame type of transistors as those used in logic LSIs and I/O circuitscan be used for the transistors used in the memory cells. (However, toensure both high speed processing and high retention time in memorycells, it is acceptable that the transistors in 3T memory cells do nothave a silicide diffusion layer and the other transistors have asilicide diffusion layer for low diffusion layer resistance.) When alogic LSI and a dynamic memory are integrated on a chip in this way, thecomplexity of the fabrication process will be drastically decreased.

The major effects of the above embodiments are as follows.

(1) Since a destructive read type dynamic memory is used, data on bitlines need not be amplified and time correspondent to tRAS is notrequired. Further, as the amplitude for the bit lines is small,precharge time is short.

(2) As a result of (1), cycle time tRC can be much shorter than inconventional dynamic memories. Taking full advantage of this feature,the pipeline pitch can be decreased by pipelining the dynamic memorylike a pipelined SRAM.

(3) If a sense amplifier based on the direct sensing scheme is used inthe dynamic memory, quicker amplification is possible. In conventionaldynamic memories, if a sense amplifier based on the direct sensingscheme should be used, it would be necessary to install an additionalamplifier for rewriting into memory cells in parallel with it. Thedynamic memory according to this invention does not require such anadditional amplifier, which means a reduction in chip area.

(4) With the above-mentioned configuration, the read latency and writelatency in a pipelined dynamic memory can be identical. This canincrease the pipeline fill rate in case both read and write accessrequests exist.

(5) The dynamic memory's refresh operation can be concealed bypipelining the dynamic memory and adding an access control circuit ACCRLexternally.

(6) When 3T memory cells are used, the above effects can be obtainedwithout using any cache 110.

According to major aspects of this invention, the read and write cycletimes for dynamic memory cells can be shortened to realize a higherspeed DRAM.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details can be made therein without departing from the spirit andscope of the invention.

What is claimed is:
 1. A semiconductor device comprising: a memory circuit including a plurality of memory cells provided at intersections of a bit line and a plurality of word lines, a row decoder coupled to the plurality of word lines, and a row address latch circuit for latching a row address at a transition point of a first clock with a predetermined cycle and supplying it to said row decoder; and an access control circuit which receives an external address supplied to a plurality of first nodes at a predetermined timing of a second clock supplied to a second node, and supplies the external address at the timing of the first clock to the row address latch circuit, wherein the cycle of the first clock is shorter than that of the second clock.
 2. The semiconductor device according to claim 1, wherein the memory circuit further includes: a read circuit for amplifying the stored signal read to the bit line from one of the plurality of memory cells and outputting it to a third node; a write circuit for writing data inputted to a fourth node, as memory data, into one of the plurality of memory cells via the bit line; and a data latch circuit connected between the third and fourth nodes, wherein the access control circuit further includes a refresh control circuit by which a first word line is selected for a predetermined time period from a first transition point of the first clock, first data read from a first memory is read by the read circuit and held by the data latch circuit, hereinafter the first word line is selected for a predetermined time period from a second transition point of the first clock after the first transition point and the first data held by the data latch circuit is written back to the first memory cell via the write circuit and the bit line in order to refresh the memory data in the first memory cell, the first word line is one of the plurality of word lines, and the first memory cell is one of the plurality of memory cells connected with the first word line.
 3. The semiconductor device according to claim 1, wherein the access control circuit further includes a refresh control circuit which supplies a first row address for reading first data from a first memory cell, one of the plurality of memory cells, to the address latch circuit at one transition point of the first clock at which the external address is not supplied, and then supplies the first row address for writing the first data into the first memory cell, to the address latch circuit at another transition point of the first clock at which the external address is not supplied.
 4. The semiconductor device according to claim 3, wherein the frequency of the second clock is half that of the first clock.
 5. The semiconductor device according to claim 4, wherein the refresh control circuit receives the external address at a rising edge of the second clock, supplies the external address to the row address latch circuit at a rising edge of the first clock corresponding to that of the second clock, and then supplies the first row address for refreshing to the row address latch circuit, at a rising edge of the first clock corresponding to a falling edge of the second clock, without receiving the external address at the falling edge of the second clock.
 6. The semiconductor device according to claim 1, wherein the access control circuit further includes a clock generating circuit which receives the second clock and generates the first clock, the memory circuit further has a clock distribution circuit which distributes the first clock into the memory circuit, and the clock generating circuit further has a phase comparator which compares the phase of the second clock with that of the first clock distributed and fed back via the clock distribution circuit.
 7. The semiconductor device according to claim 1, wherein each of the plurality of memory cells has a MISFET whose gate is connected to a corresponding word line and whose source or drain is connected with the bit line, and a capacitor connected with remaining one of the source or drain of the MISFET, wherein data is stored by the electric charge stored in the capacitor.
 8. The semiconductor device according to claim 1, wherein each of the plurality of memory cells is a non-destructive read memory cell.
 9. The semiconductor device according to claim 1, wherein the bit line is divided into a first bit line for reading and a second bit line for writing, each of the plurality of memory cells has a first MISFET whose gate is connected to a corresponding word line and whose source or drain is connected with the first bit line, a second MISFET whose source or drain is connected with the remaining one of the source or drain of the first MISFET, and a third MISFET whose gate is connected with the corresponding word line and whose source or drain is connected with the gate of the second MISFET.
 10. The semiconductor device according to claim 1, further comprising an output circuit with a first MISFET for outputting signals within the semiconductor device to the outside of it, and a logical gate circuit with a second MISFET, wherein each of the plurality of memory cells has a third MISFET, and wherein the gate-oxide thickness of the third MISFET is the same as that of the first MISFET and larger than that of the second MISFET.
 11. The semiconductor device according to claim 1, further comprising an input circuit with a first MISFET for inputting signals from the outside of the semiconductor device into it and a logical gate circuit with a second MISFET, wherein each of the plurality of memory cells has a third MISFET, and wherein the gate-oxide thickness of the third MISFET is the same as that of the first MISFET and larger than that of the second MISFET.
 12. A semiconductor device comprising: a plurality of memory cells provided at intersections of a bit line and a plurality of word lines; a read circuit for amplifying a signal read to the bit line from one of the plurality of memory cells and outputting it to a first node; a write circuit for writing data inputted to a second node, as a memory data, into one of the plurality of memory cells via the bit line; a data latch circuit connected between the first and second nodes; and a refresh control circuit by which a first word line is selected for a predetermined time period from a first transition point of a predetermined clock, first data read from a first memory cell is read by the read circuit and held by the data latch circuit, hereinafter the first word line is selected for a predetermined time period from a second transition point of the predetermined clock after the first transition point, and the first memory data held by the data latch circuit is written back to the first memory cell via the write circuit and the bit line in order to refresh the data in the first memory cell, the first word line is one of the plurality of word lines, and the first memory cell is one of the plurality of memory cells connected with the first word line.
 13. The semiconductor device according to claim 12, wherein each of the plurality of memory cells has a MISFET whose gate is connected with a corresponding word line and whose source or drain is connected with the bit line, and a capacitor connected with the remaining one of the source or drain of the MISFET, wherein data is stored by the electric charge stored in the capacitor.
 14. The semiconductor device according to claim 12, wherein each of the plurality of memory cells is a non-destructive read memory cell.
 15. The semiconductor device according to claim 12, wherein the bit line is divided into a first bit line for reading and a second bit line for writing, wherein each of the plurality of memory cells has a first MISFET whose gate is connected with a corresponding word line and whose source or drain is connected with the first bit line, a second MISFET whose source or drain is connected with the remaining one of the source or drain of the first MISFET, and a third MISFET whose gate is connected with the corresponding word line and whose source or drain is connected with the gate of the second MISFET.
 16. A semiconductor device comprising: a memory circuit which includes a plurality of memory cells provided at intersections of a plurality of bit lines and a plurality of word lines; and an access control circuit which receives an external address and a plurality of external commands indicating reading or writing at a transition point of a first clock, and supplies them to the memory circuit as an internal command and an internal address at a transition point of a second clock whose frequency is higher than that of the first clock, wherein the access control circuit further has a refresh control circuit which refreshes the plurality of memory cells at a transition point of the second clock whose timing does not allow the external command and external address to be supplied.
 17. The semiconductor device according to claim 16, wherein the frequency ratio of the first clock to the second clock is a rational number.
 18. The semiconductor device according to claim 16, wherein the external address is signals for selecting one of the plurality of memory cells. 